1. Field of the Invention
This invention relates generally to the processing of biologically generated signals and particularly to detectors of the type used to identify QRS complexes and other periodic wave forms in signals, such as electrocardiograms, commonly known as EKG signals, developed during the monitoring of living beings. Such QRS detectors find particular usefulness in heart rate monitors and similar equipment, such as controlled defibrillators, demand pacemakers, and devices synchronized to heart activity for performing injections or assisting circulation.
2. Description of the Prior Art
QRS detectors are typically used to identify QRS complexes in the presence of other biological and equipment generated signals, such as P and T waves and sinusoidal noise, such as AC line voltage noise. In heart rate monitors, the detected QRS complexes are then processed by counting or other techniques to determine the heart rate. Such monitors normally include electrodes and an amplifier for detecting the EKG; a high pass filter for enhancing the QRS complexes while minimizing the lower frequency P and T waves; a QRS detector; a pulse shaper for generating a fixed width pulse when a QRS complex is detected; and a pulse counter, together with recording and/or alarm equipment.
Conventional QRS detectors generate an output indicating the presence of a QRS complex whenever the amplitude of the filtered EKG signal exceeded the threshold value. The threshold values were originally manually selected by the technician operating the heart rate monitoring equipment and then automatic threshold circuitry was developed for generating a threshold value related to the amplitude of the filtered EKG signal. The threshold value generated in this manner was affected by changes of the polarity of the QRS complex. Full wave rectifiers were then added to the equipment, in the signal processing before the QRS detection, to avoid polarity dependence. Amplitude selection circuitry is sometimes used to filter out pulses generated by pacemakers and similar equipment.
QRS detectors with automatic threshold value generating circuitry have conventionally used a capacitor-resistor network in an integrating mode to store a slowly decaying threshold voltage value derived from the filtered and/or rectified EKG signal and transistor switching circuitry or a comparator for generating an output signal whenever the instantaneous value of the EKG signal exceeded the slowly changing threshold value stored across the capacitor. Such QRS detectors provided improved recognition of QRS complexes, particularly during circumstances of chaning QRS complex amplitudes caused, for example, by changes in quality of the connections between the electrodes and the body.
Conventional QRS detectors as described above are, unfortunately, not always able to distinguish between QRS complexes and noise, particularly sinusoidal noise, such as that caused by the AC line voltage. An improved QRS detector is shown in German published patent application DE-OS 25 45 802, Heart Signal Discriminator, which has been designed to discriminate between QRS complexes and AC noise. This circuit operates by generating a threshold value equal to the mean value of the amplitude of the EKG signal and then compares the instantaneous EKG signal against this threshold value. An output signal is then generated whenever the peak value of the instantaneous EKG signal exceeds the mean value by a predetermined amount. Such QRS detectors are still susceptible to error induced by AC noise, particularly during variations in the amplitude of the QRS complexes. For example, the threshold value may become too large to indicate the presence of QRS complexes when such signals are weak.